The present invention relates generally to surgical devices and methods and, more particularly, to minimally invasive surgical devices and methods for creating a cavity within hard or soft tissue.
Surgeons are using minimally invasive surgical techniques on an increasing basis for the treatment of a wide variety of medical conditions. Such techniques typically involve the insertion of a surgical device through a natural body orifice or through a relatively small incision using a tube or cannula. In contrast, conventional surgical techniques, typically involve a significantly larger incision and are therefore sometimes referred to as open surgery. Thus, as compared with conventional techniques, minimally invasive surgical techniques offer the advantages of minimizing trauma to healthy tissue, minimizing blood loss, reducing the risk of complications such as infection, and reducing recovery time. Further, certain minimally invasive surgical techniques can be performed under local anesthesia or even, in some cases, without anesthesia, and therefore enable surgeons to treat patients who would not tolerate the general anesthesia required by conventional techniques.
Surgical procedures often require the formation of a cavity within either soft or hard tissue, including bone. Tissue cavities are formed for a wide variety of reasons, such as for the removal of diseased tissue, for harvesting tissue in connection with a biopsy or autogenous transplant, and for implant fixation. To achieve the benefits associated with minimally invasive techniques, tissue cavities should be formed by creating only a relatively small access opening in the target tissue. An instrument or device can then be inserted through the opening and used to form a hollow cavity that is significantly larger than the access opening. Depending on the specific application, the shape of the desired cavity can be spherical, hemispherical, cylindrical, or any number of different combinations or variations of such shapes.
One important surgical application requiring the formation of a cavity within tissue is the surgical treatment and prevention of skeletal fractures associated with osteoporosis, which is a metabolic disease characterized by a decrease in bone mass and strength. The disease leads to skeletal fractures under light to moderate trauma and, in its advanced state, can lead to fractures under normal physiologic loading conditions. It is estimated that osteoporosis affects approximately 15-20 million people in the United States and that approximately 1.3 million new fractures each year are associated with osteoporosis, with the most common fracture sites being the hip, wrist and vertebrae.
An emerging prophylactic treatment for osteoporosis involves replacing weakened bone with a stronger synthetic bone substitute using minimally invasive surgical procedures. The weakened bone is first surgically removed from the affected site, thereby forming a cavity. The cavity is then filled with an injectable synthetic bone substitute and allowed to harden. The synthetic bone substitute provides structural reinforcement and thus lessens the risk of fracture of the affected bone. Without the availability of minimally invasive surgical procedures, however, the prophylactic fixation of osteoporosis-weakened bone in this manner would not be practical because of the increased morbidity, blood loss and risk of complications associated with conventional procedures. Moreover, minimally invasive techniques tend to preserve more of the remaining structural integrity of the bone because they minimize surgical trama to healthy tissue.
Other less common conditions in which structural reinforcement of bone can be appropriate include bone cancer and avascular necrosis. Surgical treatment for each of these conditions can involve removal of the diseased tissue by creating a tissue cavity and filling the cavity with a stronger synthetic bone substitute to provide structural reinforcement to the affected bone.
Existing devices for forming a cavity within soft or hard tissue are relatively complex assemblies consisting of multiple components. U.S. Pat. No. 5,445,639 to Kuslich et al. discloses an intervertebral reamer for use in fusing contiguous vertebra. The Kuslich et al. device comprises a cylindrical shaft containing a mechanical mechanism that causes cutting blades to extend axially from the shaft to cut a tissue cavity as the shaft is rotated. The shaft of the Kuslich et al. device, however, has a relatively large diameter in order to house the blade extension mechanism, and therefore it is necessary to create a relatively large access opening to insert the device into the body. The complexity of the device leads to increased manufacturing costs and may also raise concerns regarding the potential for malfunction.
U.S. Pat. No. 5,928,239 to Mirza discloses a percutaneous surgical cavitation device and method useful for forming a tissue cavity in minimally invasive surgery. The Mirza device comprises an elongated shaft and a separate cutting tip that is connected to one end of the shaft by a freely-rotating hinge, as shown in FIG. 1 hereto. The cutting tip of the Mirza device rotates outward about the hinge, thereby permitting the device to cut a tissue cavity that is larger than the diameter of the shaft. However, the Mirza device relies on rotation of the shaft at speeds ranging from 40,000 to 80,000 rpm to cause the cutting tip to rotate outward about the hinge. Such high rotational speeds can only be produced by a powered surgical drill and certainly cannot be produced by manual rotation. Thus, the Mirza device does not permit the surgeon to exercise the precise control that can be attained through manual rotation. Moreover, there may be a concern for structural failure or loosening of the relatively small hinge assembly at such a high rotational speed when operated in bone. The high rotational speed of the Mirza device may also generate excessive heat that could damage healthy tissue surrounding the cavity.
U.S. Pat. No. 6,066,154 to Reiley et al. discloses an inflatable, balloon-like device for forming a cavity within tissue. The Reiley et al. device is inserted into the tissue and then inflated to form the cavity by compressing surrounding tissue, rather than by cutting away tissue. The Reiley et al. device, however, is not intended to cut tissue, and at least a small cavity must therefore be cut or otherwise formed in the tissue in order to initially insert the Reiley et al. device.
Thus, a need continues to exist for a tissue cavitation device and method that can form tissue cavities of various shapes that are significantly larger than the access opening in the target tissue. A need also exists for a cavitation device that is of relatively simple construction and inexpensive to manufacture, that can be operated either manually or by a powered surgical drill, and that, in the case of manual operation, provides the surgeon with increased control over the size and shape of the cavity formed.
The present invention comprises an improved tissue cavitation device and method that utilizes shape-changing behavior to form cavities in either hard or soft tissue. The shape-changing behavior enables the device to be inserted into tissue through a relatively small access opening, yet also enables the device to form a tissue cavity having a diameter larger than the diameter of the access opening. Thus, the invention is particularly useful in minimally invasive surgery, and can be used for at least the following specific applications, among others: (1) treatment or prevention of bone fracture, (2) joint fusion, (3) implant fixation, (4) tissue harvesting (especially bone), (5) removal of diseased tissue (hard or soft tissue), and (6) general tissue removal (hard or soft tissue).
The cavitation device of the present invention comprises a rotatable shaft having a flexible cutting element that is adapted to move between a first shape and a second shape during the process of forming an internal cavity within tissue. The process of forming the cavity primarily involves cutting tissue as the shaft is rotated about its longitudinal axis, but those skilled in the art will appreciate that the device also can form a cavity by impacting tissue or displacing tissue as the shaft is either partially or completely rotated. The internal cavity formed by the device has a significantly larger diameter than the diameter of the initial opening used to insert the device into the tissue. The present invention also comprises flexing means for biasing the flexible cutting element to move from its first shape to its second shape. One such means comprises spring bias arising from elastic deformation of the flexible cutting element. A second such means comprises bias arising from the behavior of a thermal shape-memory alloy. A third such means comprises bias arising from centrifugal force generated as the shaft is rotated. A fourth such means comprises a tension cable that forcefully actuates the shape change of the flexible cutting element. The device of the invention can be operated by conventional surgical drills, and some embodiments also can be manually operated using a conventional T-handle. When a T-handle is used to operate the device, the T-handle also can be adapted to apply tension to the tension cable.
During minimally invasive surgery, the flexible cutting element of the cavitation device can be adapted to assume a first shape for insertion of the device into tissue through a tube placed percutaneously, thereby creating only a relatively small access opening in the tissue. Depending on the application and size, the insertion tube can be a trochar, a cannula, or a needle. As the device is inserted beyond the distal end of the insertion tube, the flexible cutting element is adapted to assume a second shape for forming a cavity in tissue upon rotation of the shaft. When it assumes the second shape, the flexible cutting element extends or projects away from the longitudinal axis of the shaft. Thus, the diameter of the cavity is greater than the diameter of the initial access opening or pilot hole. In addition to cutting, a flexible cutting element is capable of displacing and impacting tissue away from the axis of the shaft.
According to one method of the present invention, the periphery of the target tissue, such as bone, can be accessed with an insertion tube placed percutaneously, and a pilot hole can be formed in the bone with a standard surgical drill and drill bit. Next, the cavitation device of the present invention is inserted to the depth of the pilot hole and rotated. As the flexible cutting element of the device moves from its first shape to its second shape, portions of the cutting element forcefully extend away from the longitudinal axis of the shaft, thereby forming a tissue cavity. Emulsified bone can be removed through known irrigation and suction methods. In the case of bone harvesting, the abated bone is used at another surgical site to promote healing of a bony deficit or to promote joint fusion. The cavity can then be filled with a suitable bone substitute that is injectable and hardens in situ. In the case of removing and replacing osteoporotic bone, the cavity is filled with structural synthetic bone or bone cement. Since the device and methods of the present invention are minimally invasive, they can be used for the prevention of osteoporosis related fractures in individuals at high risk. Skeletal structures where osteoporosis related fractures are common include the radius, femur, and vertebral bodies.
Surgeons can create cavities of various shapes and sizes with the device and methods of the present invention. For example, cavities of various shapes and sizes can be formed by moving the cavitation device along its axis of rotation or transverse to its axis of rotation. The size and shape of the cavity also can be modified by adjusting the insertion angle of the shaft (or the insertion tube, if one is used) with respect to the tissue angle. Tissue cavities of various shapes and sizes also can be interconnected to form more complex shapes.
The objects and advantages of the present invention include simplicity, wherein a flexible cutting element eliminates the need for complex assemblies with numerous moving parts. The shape-changing behavior of the flexible cutting element enables the device to be adapted to a shape suitable for minimally invasive placement in tissue. The inherent outward forces associated with the shape change of the flexible cutting element assist in the cutting and displacement of tissue during the process of forming a cavity.